The inventive arrangements relate generally to slot antennas.
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the dielectric constant.
For the purpose of RF circuits, it is generally important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, signal reflections and inefficient power transfer can result. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting circuit performance relate to the dielectric constant (sometimes referred to as the relative permittivity or xcex5r) and the loss tangent (sometimes referred to as the dissipation factor or xcex4) of the dielectric substrate material. The dielectric constant determines the electrical wavelength in the substrate material, and therefore the electrical length of transmission lines and other components disposed on the substrate. The loss tangent determines the amount of signal loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes.
In general, the characteristic impedance of a parallel plate transmission line, such as stripline or microstrip line, is approximately equal to {square root over (L1/C1)}, where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the dielectric constant of the dielectric material(s) used to separate the transmission lines.
In conventional RF designs, a substrate material is selected that has a single dielectric constant and relative permeability value, the relative permeability value being about 1. Once the substrate material is selected, the line characteristic impedance value is generally exclusively set by controlling the geometry of the line, the slot, and coupling characteristics of the line and the slot.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate, such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors, such as copper, gold, or tantalum, which generally function as transmission lines (e.g. stripline or microstrip line or twin-line) in the frequency ranges of interest.
The dielectric constant of the selected substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is reasonably suitable for all of the various passive components, radiating elements and transmission line circuits to be formed on the board.
In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to have an electrical length of a quarter of a wavelength. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation for a given substrate. Since the physical size of the microstrip line or stripline is inversely related to the dielectric constant of the dielectric material, the dimensions of a transmission line or a radiator element can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a high dielectric constant with values such as 50 to 100. However, the use of a dielectric with a high dielectric constant will generally result in a significant reduction in the radiation efficiency of the antenna.
Antenna elements are sometimes configured as microstrip slot antennas. Microstrip slot antennas are useful antennas since they generally require less space, are simpler and are generally less expensive to manufacture as compared to other antenna types. In addition, importantly, microstrip slot antennas are highly compatible with printed-circuit technology.
One factor in constructing a high efficiency microstrip slot antenna is minimizing the power loss, which may be caused by several factors including dielectric loss. Dielectric loss is generally due to the imperfect behavior of bound charges, and exists whenever a dielectric material is placed in a time varying electromagnetic field. The dielectric loss, often referred as loss tangent, is directly proportional to the conductivity of the dielectric medium. Dielectric loss generally increases with operating frequency.
The extent of dielectric loss for a particular microstrip slot antenna is primarily determined by the dielectric constant of the dielectric space between the radiator antenna element (e.g., slot) and the feed line. Free space, or air for most purposes, has a relative dielectric constant and relative permeability approximately equal to one.
A dielectric material having a relative dielectric constant close to one is considered a xe2x80x9cgoodxe2x80x9d dielectric material as a good dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant substantially equal to the surrounding materials is used, the dielectric loss due to impedance mismatches is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip slot antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator antenna slot and the microstrip feed line exciting the slot.
Furthermore, the use of a material with a lower dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the radiation efficiency of the microstrip slot antenna. However, the use of a dielectric material having a low dielectric constant can present certain disadvantages, such as the large size of the slot antenna fabricated on a low dielectric constant substrate as compared to a slot antenna fabricated on a high dielectric constant substrate.
The efficiency of microstrip slot antennas is compromised through the selection of a particular dielectric material for the feed which has a single uniform dielectric constant. A low dielectric constant is helpful in allowing wider feed lines, that result in a lower resistive loss, to the minimization of the dielectric induced line loss, and the minimization of the slot radiation efficiency. However, available dielectric materials when placed in the junction region between the slot and the feed result in reduced antenna radiation efficiency due to the poor coupling characteristics through the slot.
A tuning stub is commonly used to tune out the excess reactance in microstrip slot antennas. However, the impedance bandwidth of the stub is generally less than both the impedance bandwidth of the radiator and the impedance bandwidth of the slot. Therefore, although conventional stubs can generally be used to tune out excess reactance of the antenna circuit, the low impedance bandwidth of the stub generally limits the performance of the overall antenna circuit.
The performance of a microstrip antenna can be optimized by improving the performance of the feed stub. A feed stub is commonly used to tune out the excess reactance of slot fed antennas, but has limited design flexibility because of the constraints imposed by a common uniform dielectric substrate. The common dielectric substrate is generally selected to obtain good transmission line characteristics. Using the invention, the dielectric substrate region across the slot as well as underlying the stub can be optimized separately from the dielectric substrate characteristics needed for good transmission line characteristics.
In addition, the stub impedance bandwidth can be improved by disposing the feed stub on a high dielectric constant material. The high dielectric region preferably includes optional magnetic particles therein for a further efficiency enhancement. By including magnetic particles in the dielectric region underlying the stub, the intrinsic impedance of the dielectric junction region disposed between the feed line and slot can be matched to the dielectric material underlying the stub. Impedance matching these regions can reduce the amount of signal distortion and ringing caused by the discontinuity.
A slot fed microstrip antenna includes an electrically conducting ground plane, the ground plane having at least one slot. A feed line transfers signal energy to or from the slot, the feed line including a stub region which extends beyond the slot. A first dielectric layer is disposed between the feed line and the ground plane, the first dielectric layer having a first set of dielectric properties including a first relative permittivity over a first region, and at least a second region of the first dielectric layer having a second set of dielectric properties. The second set of dielectric properties provide a higher relative permittivity as compared to the first relative permittivity. The stub is disposed on the second region.
The first dielectric layer preferably includes magnetic particles. At least a portion of the magnetic particles are disposed in the second region which underlies the stub. The second region can provide a relative permeability of at least 1.1.
The intrinsic impedance of the dielectric junction region disposed between the feed line and slot is impedance matched to the second region which underlies the stub. This reduces ringing and signal distortion. The intrinsic impedance of the dielectric junction region can also be impedance matched to an intrinsic impedance of an environment around the antenna. As used herein, the phrase xe2x80x9cintrinsic impedance matchedxe2x80x9d refers to an impedance match which is improved as compared to the intrinsic impedance matching that would result given the respective actual permittivity values of the regions comprising the interface, but assuming the relative permeabilities to be 1 for each of the respective regions. As noted earlier, prior to the invention, although board substrates provided a choice regarding a single relative permittivity value, the relative permeability of the board substrates available was necessarily equal nearly 1.
The first dielectric layer can comprises a ceramic material, the ceramic material having a plurality of voids, at least a portion of the voids filled with magnetic particles. The magnetic particles can comprise meta-materials. The antenna can be a patch antenna by including at least one patch radiator and a second dielectric layer, the second dielectric layer disposed between the ground plane and the patch radiator. The second dielectric layer can include a third region which provides a third set of dielectric properties including a third relative permittivity, and at least a fourth region including a fourth set of dielectric properties including a fourth relative permittivity. The fourth relative permittivity is higher as compared to the third relative permittivity, wherein the patch is disposed on the fourth region. The fourth region can include magnetic particles and provide a relative permeability of at least 1.1.
The invention can be used to impedance match the various medium interfaces provided by the antenna. For example, the intrinsic impedance of the fourth region underlying the patch can match the intrinsic impedance of an environment around the antenna. The intrinsic impedance of the dielectric junction region disposed between the feed line and slot can match the intrinsic impedance of the fourth region and/or the second region underlying the stub.
The antenna can include multiple patches, such as a first and a second patch radiator, the first and second patch radiators being separated by a third dielectric layer. The third dielectric layer can be structured in accordance with the principles applied to the first and second dielectric layers as explained above.